Blade for a Wind Energy Plant Comprising Segmented Conductor Means for Conducting Lightning

ABSTRACT

A method of manufacturing a fibre-reinforced blade for a wind energy plant is presented, which blade is configured with at least a blade shell and means for conducting to a lightning current to an earth connection. Novel aspects of the method comprise that the blade is provided with segmented conductor means that are configured for conducting a lightning current outside the blade to the means for conducting to earth; and wherein the conductor means are distributed and secured at the external surface of the blade shell in such a manner that the conductor means are essentially flush with the external surface of the blade shell. When the segmented conductor means are distributed and attached at the external surface of the blade shell, the lightning current will not have to be conducted through the conductor means, but rather they are conducted in a ionised passage in the air above the conductor means.

The invention relates to a method of manufacturing a fibre-reinforcedblade for a wind energy plant, said blade being configured with at leasta blade shell and conductor means adapted for conducting a lightningcurrent to an earth connection. The invention also relates to a bladefor a wind energy plant.

The ever increasing development within the field of wind energy plantsgoes in the direction of increasingly larger units with increasinglyhigher towers and longer blades, whereby the risk of being struck bylightning is increased. The plants are typically safeguarded withrespect to lightning strikes in such a manner that a lightning current,if any, is captured and conducted to earth in a controllable manner,thereby avoiding damage to sensitive elements of the wind energy plant.One of the most widely used methods of lightning-proofing consists inmounting of one or more so-called lightning receptors which areelectrically conductive elements that are arranged eg at the blade tipsand connected to internal lightning conductors in the blades. This isknown eg from EP 0 783 629. A similar method is taught in U.S. Pat. No.6,457,943, where a wind turbine blade is structured with long parts ofcarbon-fibre material throughout the entire length of the blade. Thecarbon fibre which is electrically conductive thus acts as lightningreceptor, and the lightning current is thus conducted through thematerial and downwards in interiorly arranged lightning conductors.Thus, that method concerns the entire blade structure and makesrequirements to, on the one hand, the use of carbon fibre throughoutmajor parts of the blade—which is not always desirable—and, on the otherhand, a certain thickness of the carbon fibre parts in order for thematerial to be able to conduct the lightning current without sufferingensuing damage.

Typically the blades are the part of the wind energy plant thatprotrudes farthest into the air and which is hence exposed to a largerisk of being struck by lightning. Wind energy plants are arranged inlarge numbers, eg at sea in the form of wind farms, which entails thedrawback that service and maintenance becomes both quite costly andquite cumbersome due to the weather conditions and the pooraccessibility. Salt from the sea air also deposits on eg the blades andmake them electrically conductive, thereby increasing the risk oflightning striking.

It is suggested in WO 01/77527 to adhere strips of cupper tape onto theblades and connect the strips to receptors. The strips are intended forbeing able to conduct a lightning current to a receptor, from where itis conveyed on to earth via a cable. This involves that the strips mustpossess sufficient conductivity to be able to carry a lightning currentthat may be in excess of 50 kA. It must be possible to replace thestrips following lightning strikes due to damage caused by powerfulheating due to the lightning current, which entails both monitoring ofthe wind energy plant, standstill during replacement as well as highcosts. Moreover, there is a risk of the strips loosening and henceadversely influencing the aerodynamic properties of the blade.Alternatively the strips must be of a very heavy configuration, whichwould entail undesirable additional weight on the blade. When longblades are to be made, one of the design problems is precisely to reducethe weight, since per se it constitutes a load not only on the blade,but also on the hub, the nacelle and the tower.

Airplanes are struck by lightning as well and consequently they must belightning-proofed as well. Airplanes are provided with radar equipment,ia for navigation purposes, that is typically arranged in the nose ofthe plane in order to be able to look ahead. When radar equipment isarranged in the nose, the nose is not constructed from aluminium likethe rest of the plane, but rather from a plastics material since,otherwise, the radar would be unable to look through the nose. Therebeing a risk of lightning striking also in or via the nose of theairplane, it has to be lightning proofed; however, in a manner so as notto disturb the radar. Since the 1960ies it has been known to carry outthe lightning proofing via so-called lightning diverter strips that mayhave various configurations. One example will appear from U.S. Pat. No.4,237,514, wherein a feedstock material provided with aluminium powderis adhered in strips to ia the nose of an airplane. The aluminium powderdoes not constitute a continuous conductor, but rather interrupted orsegmented conductive particles. When those metallic, each separatelyconductive particles are exposed to a large voltage field due to alightning strike, the particles short-circuit and a current-conductingionised passage is formed in the air above the particles, in which thelightning current can be conducted to eg the metal hull of the plane.Instead of aluminium powder, U.S. Pat. No. 4,506,311 teaches button orclub-shaped pieces of metal that are separately incorporated in afeedstock material that is shaped into a band. Both band and strips areintended for being mounted exteriorly on the plane nose, where they arearranged to project symmetrically from the nose tip. Such locationyields good protection, but also involves some degree of aerodynamicdisturbance. On a blade for a wind energy plant, the arrangement ofbands and strips on top of the aerodynamic profile of the blade willentail an undesired adverse effect on the efficiency and performance ofthe plant. Bands or strips will in this manner also be sources of noise,which will limit where and how close plants can be deployed. Besides,bands or strips of metal or metal grids possess a significantlydifferent elasticity than the commonly used fibres forfibre-reinforcement of the blade shell. They are considerably more rigidand are hence exposed to large tensions due to the quite high stressstrains to which the blade is exposed in practice, and therefore suchbands or strips are susceptible to crack formation due to fatigue.

It is an object of the invention to provide a blade and a method ofmanufacturing a blade for a wind energy plant, where lightning strikesin the blade can be conducted to earth in a safe manner by means ofconductor means that can be configured such that lightning strikes donot necessitate exchange and such that they influence the weight andaerodynamic properties of the blade minimally.

Other objects will appear from the description.

Novel aspects of the method according to the invention involve that themethod comprises that the blade is provided with segmented conductormeans that are configured for conducting a lightning current outside theblade to the means for conducting to earth, and wherein the conductormeans are distributed and secured at the external surface of the bladeshell in a manner to enable the conductor means to be essentially inlevel with the external surface of the blade shell. The conductor meansbeing essentially in level with the external surface of the blade shellmeans that the influence exerted on the aerodynamic properties of theblade is minimal. When the segmented conductor means are distributed andsecured at the external surface of the blade shell, the lightningcurrent will not have to be conducted through the conductor means, butrather be conduced in an ionised passage in the air above the conductormeans. Thereby the conductor means can be realised with very low weight,since they are not to be able to tolerate the lightning current. Herebythe weight of the blade is also influenced minimally. As the conductormeans are not to tolerate the lightning current, the heating is limitedand it follows that the damage inflicted on the conductor means isminimal. Since the conductor means can be configured with low weight,they can also be configured as eg duplicate means without the weightbecoming a problem, whereby it is possible to create a spare capacity,thereby providing satisfactory longevity. The conductor means beingsegmented, their rigidity is limited. Since precisely the conductormeans are segmented, they will have no propensity to crack; rather theywill be able to tolerate large stress strains.

A preferred embodiment of the method comprises the steps:

-   -   a) arrangement of segmented conductor means on a mould;    -   b) laying of fibres on the mould, including on top of the        conductor means;    -   c) attachment of the fibres and the conductor means with resin.

Hereby it is accomplished that the conductor means are cast integrallywith the blade and are therefore unable to disengage under the influenceof the weather conditions. Moreover the conductor means can be arrangedvery accurately in level with the external surface of the blade shell,thereby minimising the influence on the aerodynamic properties of theblade.

A convenient embodiment comprises application of a substance on themould, including gel-coat, resin, primer or release agent. This can bedone either before or after step a). Hereby it is possible ia for theconductor means to be integrated to a particular extent in the surfaceof the blade. Blades for wind energy plants are often moulded by use ofconventional manual laying or by a VARTM method (Vacuum Assisted ResinTransfer Moulding) or other suitable method. It is a common feature ofthe methods that female moulds are employed in which the outermost layerof the blade is the first one formed with either a layer of gel-coat orof the resin that is caused by partake in the blade as such.Alternatively the very first substance to be applied to the mould is aprimer or a release agent.

A further preferred embodiment may comprise sanding or polishing of theblade to expose the conductor means. Hereby the performance of theconductor means is ensured as they need a free surface which is exposedto the ambient air.

According to yet a preferred embodiment the conductor means can bearranged in a pre-manufactured band made of an electricallynon-conductive material, including of a thermoplastic material. Herebythe conductor means can be finished and tested prior to use, whichensures uniformity and prevents defects and, likewise, it simplifieshandling.

According to an alternative embodiment the conductor means can bearranged in an elongate, bag-like band, which band is configured such asto be penetrated by resin. The band with conductor means can thus becast integrally with the blade shell in such a manner that the conductormeans can be secured by means of resin. Following moulding the conductormeans must be exposed, eg by sanding.

A further preferred embodiment may comprise that the segmented conductormeans and/or the band is prior to step b) during moulding fixated to themould by adhesive means, including double-adhesive tape. In this mannerthe conductor means can be secured in a precise position during mouldingeven on inclined or curved surfaces.

According to yet a preferred embodiment the conductor means can beelectrically conductive particles having an expanse of between 0.05 and10 mm, including preferably between 1 and 8 mm. These sizes bring aboutgood performance of the conductor means simultaneously with low weightof the conductor means.

According to a further alternative embodiment the method may comprisearrangement of at least one masking on the mould, following which themasking is provided with a mixture of gel-coat and electricallyconductive particles. This embodiment is very flexible with regard topositioning of the electrically conductive particles, and likewise theparticles become very well integrated in the blade shell surface. Anoptically pleasant configuration of the blade is provided, which is alsoa positive feature in respect of aerodynamics and hence of theefficiency and performance of the wind energy plant.

According to yet an alternative embodiment the electrically conductiveparticles may be admixed with electrically non-conductive particles, egceramic particles, colour pigments, etc. This can be used to advantageto create suitable distance between the electrically conductiveparticles, ie with a view to achieving and safeguarding suitablesegmentation so as to avoid the occurrence of a continuous conductor.The use may also bring about an optical effect eg to indicate where theelectrically conductive particles are arranged on the blade, if it isdesired eg to see that from the ground when a wind energy plant is inoperation.

According to a preferred embodiment the particles can be flat andelongate and of a length of between 2 and 10 mm and a transverse expansebetween 1 and 5 mm. According to a further preferred embodiment theparticles can be flat and essentially circular with a diameter between 2and 10 mm and a thickness between 0.1 and 1 mm. In both cases abeneficial effect is accomplished with a view to producing a ionisedpassage in the air above the particles for conducting lightning current.

According to yet a preferred embodiment the conductor means can be metalshavings that are preferably made by planing, milling or turnery. Themetal shavings can be made very uniformly and at low costs in suitabledimensions.

According to an alternative embodiment the method may comprise that theblade shell is configured with a number of recesses, in which recessesthe conductor means are fixed. In this manner the configuration can besuch that the conductor means can easily be replaced in the rare eventthat this should be necessary.

According to a further preferred embodiment the conductor means can bedistributed in at least one path, which path has a width comprisedwithin 3 and 50 mm, including between 5 and 20 mm, including preferablybetween 8 and 12 mm. Such dimensions mean that the conductor means canbe engaged by lightning current a great number of times.

The at least one path can preferably be arranged essentiallytransversally to the longitudinal expanse of the blade and extendessentially from the fore edge of the blade to the aft edge of theblade. Alternatively the conductor means can be arranged in star-array,including with a receptor arranged in the centre and connected to meansfor conducting to earth. Other patterns are also an option.

According to yet a preferred embodiment the conductor means can bedistributed in at least one path which is arranged essentiallytransversally to the longitudinal expanse of the blade, and spanning atleast one main laminate in the blade shell, said main laminatecomprising electrically conducting fibres. Hereby it is accomplishedthat the conductor means shield the electrically conducting fibres,whereby the risk of lightning strikes therein is reduced.

According to yet an alternative embodiment the conductor means can bewires of metal laid out essentially in the same direction and separatedby predetermined spacings. Such conductor means can be manufactured witha high degree of precision and at relatively low costs.

The conductor means may preferably be made of metal, including brass,nickel, cupper, brass coated with nickel or varnished copper. Metals arepreferably used that have a limited tendency towards oxidation uponcontact with ambient air. Moreover, metals are resistant to the wear towhich the blade is exposed in practice.

According to yet a preferred embodiment, the means for conducting toearth may comprise at least one receptor arranged at the blade surface.The receptor is suitable for collecting the lightning current at thesurface of the blade and conveying it on interiorly of the blade to eg acable for conduction to an earth connection. A receptor can also bearranged at the blade tip, where the risk of lightning strikes is high,since the receptor is able to tolerate comparatively many strikes.

The receptor can also be arranged in a recess in the blade, said recessbeing essentially encircled by conductor means. Hereby a lightningcurrent is transferred to the receptor, from where it can be conductedon to an earth connection.

The invention also comprises a blade for a wind energy plant, said bladecomprising a fibre-reinforced blade shell and means for conducting toearth adapted for conducting a lightning current to an earth connection.Novel aspects of the blade involve that the blade is provided withsegmented conductor means configured for conducting a lightning currentoutside the blade to the means for conducting to earth, and wherein theconductor means are preferably distributed and secured at the exteriorsurface of the blade shell in such a manner that the conductor means areessentially aligned with the external surface of the blade shell.

Hereby a blade is accomplished that presents the same advantages asoutlined above for a blade manufactured on the basis of a methodaccording to the invention, including that the blade is able to toleratea number of lightning strikes, the lightning current being conducted ina ionised passage in the air above the conductor means; and that theconductor means may have low weight; that the conductor means do notcrack, etc.

A preferred embodiment comprise that the conductor means can be arrangedin a number of paths that extend from a receptor arranged at the surfaceof the blade, said receptor being connected to the means for conductingto earth. Hereby a high degree of probability is provided that alightning strike will be captured by the conductor means, from where thelightning current can be conducted safely to the receptor and on to anearth connection.

According to a further embodiment the conductor means can be secured ina recess in the surface of the blade shell. The conductor means may thusbe mounted from the outside, but yet be caused to align with theexterior surface of the blade shell, whereby the influence on theaerodynamic properties of the blade is minimal.

According to yet a preferred embodiment the conductor means may be castintegrally in the surface of the blade shell. Hereby safe attachment isaccomplished which can also be performed with minimal impact on theaerodynamic properties of the blade.

According to a further preferred embodiment the conductor means maycomprise an essentially evenly distributed layer of metal shavings.Hereby both low costs and long longevity is provided. In case some ofthe shavings were to be burnt off, there may be many alternative optionsavailable for short-circuit paths via other shavings, meaning that theconductor means are functional even in slightly damaged state.

In the following the invention is described by means of figures thatteach exemplary embodiments of the invention:

FIGS. 1 a-b show sections of a blade for a wind energy plant comprisingconductor means;

FIG. 2 shows a sectional view of a mould provided with conductor means;

FIG. 3 shows a section of a mould provided with conductor means and alayer of gel coat;

FIG. 4 is a sectional view of a mould provided with a masking;

FIG. 5 is a sectional view of a mould with a pre-manufactured band withconductor means;

FIG. 6 is a sectional view of a mould provided with conductor means anda layer of gel-coat;

FIG. 7 is a sectional view of a mould provided with a bag-like bandcontaining conductor means;

FIG. 8 is a sectional view of a mould, see FIG. 7;

FIG. 9 is a sectional view of a blade for a wind energy plant comprisingconductor means;

FIG. 10 shows a manufacture of segmented conductor means;

FIG. 11 shows a band of segmented conductor means.

FIGS. 1 a and 1 b show a blade for a wind energy plant that comprises ablade shell 1 with reinforcements 2. The blade shell 1 comprises anexternal surface 3 which is provided with segmented conductor means 4.The conductor means 4 are arranged in paths and connected to means forconducting to earth. In FIGS. 1 a and 1 b the conductor means 4 arearranged such as to project from a receptor 5 which is connected to notshown means for conducting to earth that can be connected to an earthconnection. In FIG. 1 a the conductor means 4 extend essentiallytransversally to the blade 1 between its leading and aft edges. In FIG.1 b the conductor means 4 are arranged in star-array. In the embodimentsshown in FIGS. 1 a and 1 b, the conductor means 4 will have anantenna-like effect in relation to any electrically conducting fibres inthe blade shell 1.

FIG. 2 shows a mould 6 with an inside 7 that is to form the aerodynamicprofile of a blade. On its one side, a piece of double-adhesive tape 15is provided with electrically conductive particles that are distributedto constitute segmented conductor means 4. The tape 15 is adhered ontothe inside 7, following which the blade shell can be cast in a usualmanner. The onset of the moulding process will appear from FIG. 3, wherethe inside 7 of the mould 6 is coated with a tape 15 that securesconductor means 4 in the form of electrically conductive particles andthat are coated with gel-coat 8. Following finishing and curing theconductor means 4 can be exposed by removal of the tape 15, which will,following discharge from the mould, face towards the exterior surface ofthe blade. The electrically conductive particles will be cast integrallywith the blade and are hence secured when the tape 15 is removed. Inorder to ensure an even and smooth surface the area where the particlesare located can be sanded and polished.

FIG. 4 shows a mould 6 with an inside 7, on which a self-arrangingmasking 9 is located that has a predefined width 10 and thickness 11. Inprinciple the masking 9 constitutes one or more moulds of low height,into which eg a mixture 16 of gel-coat and electrically conductiveparticles can be charged, which are thereby caused to constituteconductor means 4. Alternatively it is possible to apply a thin layer ofadhesive in the masking 9, following which electrically conductiveparticles can be sprinkled in a suitable layer, and following which themasking 9 is removed and the mould 6 is ready for gel-coat.

FIG. 5 shows a mould 6 with an inside 7, on which a pre-manufacturedband 17 is arranged that contains conductor means 4. From FIG. 6 it willappear how the band 17 is arranged on the inside 7 and coated withgel-coat 8. Following application of gel-coat, a not shown fibre layingcan be fitted following which resin is injected. Following finishing andcuring the band 17 will be flush with the exterior surface of the bladeshell.

FIG. 7 shows a mould 6 with an inside 7, on which a bag-like band 18 isarranged which can be penetrated by gel-coat, resin, etc., and whichcontains conductor means 4. The band 18 has adhesive side faces 19 thatcan keep the band 18 securely to the inside 7 of the mould 6. FIG. 8shows how the band 18 with side faces 19 is deposited on the inside 7and coated with gel-coat 8 that penetrates into the band 18 and hencealso cast into the conductor means 4. Following finishing and curing theband 18 will be flush with the exterior surface of the blade shell. Forinstance by sanding, the conductor means, which are cast integrally,will be exposed and hence be in level with the exterior surface of theblade shell.

FIG. 9 shows a blade for a wind energy plant comprising a blade shell 1with reinforcements 2. The blade shell 1 comprises an external surface 3provided with segmented conductor means 4. The conductor means 4 arearranged in paths and project from a receptor 5 which is connected tonot shown means for conducting to earth that can be connected to anearth connection. In FIG. 9 the conductor means 4 extend essentiallytransversally to the blade 1 between its leading and aft edges. In thatembodiment the blade shell 1 comprises two main laminates 20 thatcontain electrically conductive fibres, such as eg carbon fibres orsteel fibres. The uppermost main laminate 20 has a transverse expansewhose delimitations are indicated by lines 21. As will appear aconductor means 4 extends from a receptor 5 and transversally to a mainlaminate 20, whereby it is shielded from lightning strikes, since anantenna effect of the conductor means is accomplished. In the shownembodiment the receptor 5 is arranged at a distance from the mainlaminate 20.

FIG. 10 shows a metal plate 22 provided with a number of recesses 23that are arranged in paths. The recesses can be made by laser cutting,blanking, punching, etc. Following configuration of the recesses, theyare filled with eg a polymer material, following which the plate is cutapart as indicated by dotted lines 24. Hereby a band 27 is produced asshown in FIG. 11. As will appear from the Figure the band consists of anumber of cross-like segments 25 that are arranged at a given distancefrom each other and that are kept together by a polymer material 26.Thus a band 27 with segmented conductor means is manufactured in asimple manner.

By segmentation of the conductor means 4 a space is provided betweenthem which is comprised within the range of from 0.1 to 5 mm, includingpreferably from 0.3 to 1.5 mm.

It will be understood that the invention as disclosed in the presentdescription with figures can be modified or changed, while continuing tobe comprised by the protective scope conferred by the following claims.

1. A method of manufacturing a fibre-reinforced blade for a wind energyplant, said blade being configured with at least a blade shell and meansfor conducting to earth adapted for conducting a lightning current to anearth connection; and wherein the method is characterised in comprisingthe steps of distributing and attaching segmented conductor meansessentially flush with the external surface of the blade shell forguiding a lightning current outside the blade to the means forconducting current to earth.
 2. A method according to claim 1,characterised in comprising the steps: a) arrangement of segmentedconductor means on a mould; b) laying of fibres on the mould, includingon top of the conductor means; c) attachment of the fibres and theconductor means by resin.
 3. A method according to claim 2,characterised in comprising application of a substance onto the mould,including gel-coat, resin, primer or release agent.
 4. A methodaccording to claim 1, characterised in comprising sanding or polishingof the blade for exposing the conductor means.
 5. A method according toclaim 1, characterised in that the conductor means are arranged in apre-manufactured band made of an electrically non-conductive material,including of a thermoplastic material.
 6. A method according to claim 1,characterised in that the conductor means are arranged in an elongatebag-like band, said band being configured to be penetrated, including byresin and/or gel-coat.
 7. A method according to claim 2, characterisedin comprising that the segmented conductor means and/or the band are,prior to step b), during moulding, fixated to the mould by adhesivemeans, including double-adhesive tape.
 8. A method according to claim 1,characterised in that the conductor means are electrically conductiveparticles that have an expanse of between 0.05 and 10 mm, includingpreferably between 1 and 8 mm.
 9. A method according to claim 2,characterised in comprising arrangement of at least one masking on themould, following which a mixture of a polymer material, includinggel-coat, and electrically conductive particles are applied in themasking.
 10. A method according to claim 8, characterised in that theelectrically conductive particles are mixed with electricallynon-conductive particles, eg ceramic particles, colour pigments, etc.11. A method according to claim 8, characterised in that the particlesare flat and elongate with a length of between 2 and 10 mm and atransverse expanse of between 1 and 5 mm.
 12. A method according toclaim 8, characterised in that the particles are flat and essentiallycircular with a length of between 2 and 10 mm and a thickness of between0.1 and 1 mm.
 13. A method according to claim 1, characterised in thatthe conductor means are metal shavings preferably made by planing,milling or turnery.
 14. A method according to claim 1, characterised incomprising that the blade shell is configured with a number of recesses,in which recesses the conductor means are secured.
 15. A methodaccording to claim 1, characterised in that the conductor means aredistributed in at least one path, said path having a width of between 3and 50 mm, including between 5 and 20 mm, including preferably between Band 12 mm.
 16. A method according to claim 15, characterised in that theat least one path is arranged essentially transversally to thelongitudinal direction of the blade and extends, essentially from thefore edge of the blade to the aft edge of the blade.
 17. A methodaccording to claim 15, characterised in that the conductor means aredistributed in at least one path which is arranged essentiallytransversally to the longitudinal direction of the blade, and spanningat least one main laminate in the blade shell, which main laminatecomprises electrically conductive fibres.
 18. A method according toclaim 1, characterised in that the conductor means are wires of metallaid out essentially in the same direction and distributed withpredetermined spacings.
 19. A method according to claim 1, characterisedin that the conductor means are preferably made of metal, includingbrass, nickel, copper, brass coated with nickel or varnished copper. 20.A method according to claim 1, characterised in that the means forconducting to earth comprises at least one receptor arranged at thesurface of the blade.
 21. A method according to claim 20, characterisedin that the receptor is arranged in a recess in the blade, said recessbeing essentially encircled by conductor means.
 22. A blade for a windenergy plant, said blade comprising a fibre-reinforced blade shell andmeans for conducting to earth adapted for conducting a lightning currentto an earth connection, wherein the blade is provided with segmentedconductor means configured for conducting a lightning current outsidethe blade to the means for conducting to earth, and wherein the blade ischaracterised in that the conductor means are essentially distributedand secured at the external surface of the blade shell in such a mannerthat the conductor means are essentially flush with the external surfaceof the blade shell.
 23. A blade according to claim 22, characterised inthat the conductor means are arranged in a number of paths that extendfrom a receptor arranged at the surface of the blade, said receptorbeing connected to the means for conducting to earth.
 24. A bladeaccording to claim 22, characterised in that the conductor means aresecured in a recess at the surface of the blade shell.
 25. A bladeaccording to claim 22, characterised in that the conductor means arecast integrally with the surface of the blade shell.
 26. A bladeaccording to claim 25, characterised in that the conductor meanscomprise an essentially evenly distributed layer of metal shavings.